Hi, On Fri, 2 Oct 2015 23:31:09, Eric Botcazou wrote: > > It's related to a known difficulty with alignment and inheritance, and other > languages are affected by variant of it, see e.g. PR c++/37798. >
I doubt that this is still an issue with the current trunk. I tried the test case from the pr37798 on x86_64, and now the object is 8-aligned, but the object was only 4-byte aligned according to the comments in the bugzilla at that time. If the middle-end would not know the alignment by means of get_object_alignment that would break the tsan instrumentation immediately, because the tsan library only works for correctly aligned data accesses. BTW: It is easy to list all gcc-6 regressions, but is there also a way to query the bugzilla for old and believed-to-be unfixable bugs? I mean something like that one is technically not a regression, but it would be good to locate forgotten trackers, and revive them from time to time... >> You remember, when I removed the TYPE_ALIGN_OK handing (initially it wasn't >> clear to me that it's entire use is only to make Ada happy), all Ada tests >> continued to pass, even on ARM. BTW: You promised me last year to give me >> an example where that makes a difference. > > I know, but that's low priority, sorry. You can probably browse the 2004 > archives and find one (or a sketch of one); that being said, gigi was a bit > changed since then so this could as well be obsolete. > I digged a bit, but did not find any hint for Ada test cases though. This change introduced TYPE_ALIGN_OK to get_inner_reference for the first time. I did not find the discussion for that in the archives: ------------------------------------------------------------------------ r66465 | kenner | 2003-05-05 00:09:48 +0200 (Mo, 05. Mai 2003) | 6 Zeilen * expr.c (store_field): Don't clobber TEMP in shift: it might be a variable. (get_inner_reference): Don't go through a VIEW_CONVERT_EXPR whose purpose is to step up the alignment. (expand_expr, case ADDR_EXPR): Force LO_SUM into memory, just like REG. And this change rewrote the code to the form that caused a regression, that was fixed later by you: ------------------------------------------------------------------------ r91511 | rth | 2004-11-30 04:52:37 +0100 (Di, 30. Nov 2004) | 14 Zeilen * expr.c (get_inner_reference): Handle REAL/IMAGPART_EXPR. (handled_component_p): Likewise. * alias.c (can_address_p): Reformat and simplify. Handle REAL/IMAGPART_EXPR. Do not disable addressability based on alias set zero. * fold-const.c (build_fold_addr_expr_with_type): Remove duplicate check for REAL/IMAGPART_EXPR. * gimplify.c (gimplify_compound_lval): Likewise. * tree-cfg.c (verify_expr): Likewise. * tree-gimple.c (is_gimple_addressable, get_base_address): Likewise. * tree-nested.c (build_addr, convert_nonlocal_reference): Likewise. (convert_local_reference): Likewise. * tree-ssa-loop-ivopts.c (prepare_decl_rtl): Likewise. That one was discussed here: https://gcc.gnu.org/ml/gcc-patches/2004-11/msg02652.html aka pr15289, the corresponding test case was added to the test suite later. But there was no Ada problem reported at that time. Bernd.
